Document the NCDD-backed devcontainer workflow

[vitormattos]

Summary

Document LibreSign's NCDD-backed Dev Container workflow in the canonical Developer Manual.

Changes

• describe the Dev Container as a thin adapter over LibreCodeCoop/nextcloud-docker-development;
• explain that the LibreSign checkout is mounted as the app while NCDD owns the generic runtime;
• document isolated worktree behavior;
• remove any need for global Docker cleanup;
• keep runtime versions out of the manual and point contributors to the repository configuration as the source of truth;
• retain the manual Nextcloud setup path for contributors who do not use Dev Containers.

Companion documentation for LibreSign/libresign#9010 and LibreSign/libresign#8988.